Quantum Mechanical Modeling of Sugar Thermochemistry

The recently developed homodesmotic hierarchy for hydrocarbons is extended to include oxygen so that accurate thermochemical quantities for sugars and sugar polymers may be computed with relatively small computational cost. In particular, the method will allow for the determination of heats of formation, which can be used to determine bond strengths important in the decomposition of sugars in, for example, the pyrolysis of biomass. This chapter includes a brief review of the current methodology for calculating thermodynamic properties using electronic structure methods and a description of the proposed extensions. Preliminary results using the lowest members of the hierarchy give a standard heat of formation value of β-D-glucopyranose-gg to be approximately 250 to 260 kcal/mol. These results are promising, and future work will include the calculation of highly accurate building blocks on which this method is based

Tris(oxazolinyl)boratomagnesium-Catalyzed Cross-Dehydrocoupling of Organosilanes with Amines, Hydrazine, and Ammonia

We report magnesium-catalyzed cross-dehydrocoupling of Si–H and N–H bonds to give Si–N bonds and H2. A number of silazanes are accessible using this method, as well as silylamines from NH3 and silylhydrazines from N2H4. Kinetic studies of the overall catalytic cycle and a stoichiometric Si–N bond-forming reaction suggest nucleophilic attack by a magnesium amide as the turnover-limiting step

Quantum Mechanical Modeling of Sugar Thermochemistry

The recently developed homodesmotic hierarchy for hydrocarbons is extended to include oxygen so that accurate thermochemical quantities for sugars and sugar polymers may be computed with relatively small computational cost. In particular, the method will allow for the determination of heats of formation, which can be used to determine bond strengths important in the decomposition of sugars in, for example, the pyrolysis of biomass. This chapter includes a brief review of the current methodology for calculating thermodynamic properties using electronic structure methods and a description of the proposed extensions. Preliminary results using the lowest members of the hierarchy give a standard heat of formation value of β-D-glucopyranose-gg to be approximately 250 to 260 kcal/mol. These results are promising, and future work will include the calculation of highly accurate building blocks on which this method is based.Reprinted (adapted) with permission from Computational Modeling in Lignocellulosic Biofuel Production, Chapter 9 (ACS Symposium Series 152) (2010): 179, doi:10.1021/bk-2010-1052.ch009. Copyright 2010 American Chemical Society.</p

Chemical Pressure Schemes for the Prediction of Soft Phonon Modes: A Chemist’s Guide to the Vibrations of Solid State Materials

The vibrational modes of inorganic materials play a central role in determining their properties, as is illustrated by the importance of phonon–electron coupling in superconductivity, phonon scattering in thermoelectric materials, and soft phonon modes in structural phase transitions. However, the prediction and control of these vibrations requires an understanding of how crystal structure and the stiffness of interatomic interactions are related. For compounds whose relationships between bonding and structure remain unclear, the elucidation of such structure–property relationships is immensely challenging. In this Article, we demonstrate how the Chemical Pressure (CP) approach can be used to draw visual and intuitive schemes relating the structure and vibrational properties of a solid state compound using the output of DFT calculations. We begin by illustrating how phonon band structures can validate the DFT-CP approach. For some intermetallic crystal structures, such as the Laves phases, the details of the packing geometries make the resulting CP scheme very sensitive to assumptions about how space should be partitioned among the interatomic contacts. Using the Laves phase CaPd₂ (MgCu₂ type) as a model system, we demonstrate how the phonon band structure provides a reference against which the space-partitioning method can be refined. A key parameter we identify is the ionicity of the crystal structure: the assumption of some electron transfer from the Ca to the Pd leads to a close agreement between the CP distribution and the major features of its phonon band structure. In particular, atomic motions along directions of positive CP (indicative of overly short interatomic distances) contribute to high frequency modes, while those along negative CPs (corresponding to overly long distances) make up the lowest frequency modes. Finally, we apply this approach to Nb₃Ge (Cr₃Si type) and CaPd₅ (CaCu₅ type), for which low-frequency phonon modes correlate with superconductivity and a rich variety of superstructures, respectively. Through these examples, CP analysis will emerge as a means of predicting the presence of soft phonon modes in a crystal structure and a guide to how elemental substitutions will affect the frequencies of these modes

Tris(oxazolinyl)boratomagnesium-Catalyzed Cross-Dehydrocoupling of Organosilanes with Amines, Hydrazine, and Ammonia

We report magnesium-catalyzed cross-dehydrocoupling of Si–H and N–H bonds to give Si–N bonds and H2. A number of silazanes are accessible using this method, as well as silylamines from NH3 and silylhydrazines from N2H4. Kinetic studies of the overall catalytic cycle and a stoichiometric Si–N bond-forming reaction suggest nucleophilic attack by a magnesium amide as the turnover-limiting step.Reprinted (adapted) from Journal of the American Chemical Society 133 (2011): 16782, doi: 10.1021/ja207641b. Copyright 2011 American Chemical Society.</p

Filling in the holes: Structural and magnetic properties of the chemical pressure stabilized LnMnxGa3 (Ln = Ho-Tm; X \u3c 0.15)

Single crystals of LnMnxGa3 (Ln = Ho-Tm; x \u3c 0.15) were grown from a Ga self-flux. These compounds crystallize in a variant of the AuCu3 structure type where Mn partially occupies the Ga6 octahedral holes. Introduction of the Mn guest atoms allows for modulation of the structures and magnetic properties of their hosts: While TmGa3 orders antiferromagnetically at ∼4.2 K, TmMnxGa3 (x = 0.05, 0.10) remains paramagnetic down to 1.8 K. Ho and Er analogs order antiferromagnetically, with effective moments and Néel temperatures, respectively, decreasing and increasing as a function of Mn concentration. DFT-chemical pressure analysis elucidates the trends in the stability of LnGa3 AuCu3-type phases and their stuffed derivatives. Guest atom insertion supports expansion of the filled octahedra, allowing the relief of negative chemical pressures in the surrounding Ga-Ga contacts. © 2013 American Chemical Society

Chemical Pressure Maps of Molecules and Materials: Merging the Visual and Physical in Bonding Analysis

The characterization of bonding interactions in molecules and materials is one of the major applications of quantum mechanical calculations. Numerous schemes have been devised to identify and visualize chemical bonds, including the electron localization function, quantum theory of atoms in molecules, and natural bond orbital analysis, whereas the energetics of bond formation are generally analyzed in qualitative terms through various forms of energy partitioning schemes. In this Article, we illustrate how the chemical pressure (CP) approach recently developed for analyzing atomic size effects in solid state compounds provides a basis for merging these two approaches, in which bonds are revealed through the forces of attraction and repulsion acting between the atoms. Using a series of model systems that include simple molecules (H₂, CO₂, and S₈), extended structures (graphene and diamond), and systems exhibiting intermolecular interactions (ice and graphite), as well as simple representatives of metallic and ionic bonding (Na and NaH, respectively), we show how CP maps can differentiate a range of bonding phenomena. The approach also allows for the partitioning of the potential and kinetic contributions to the interatomic interactions, yielding schemes that capture the physical model for the chemical bond offered by Ruedenberg and co-workers

Putting ScTGa₅ (T = Fe, Co, Ni) on the Map: How Electron Counts and Chemical Pressure Shape the Stability Range of the HoCoGa₅ Type

We explore the factors stabilizing one member of the diverse structures encountered in Ln–T–E systems (Ln = lanthanide or similar early d-block element, T = transition metal, E = p-block element): the HoCoGa₅ type, an arrangement of atoms associated with unconventional superconductivity. We first probe the boundaries of its stability range through the growth and characterization of ScTGa₅ crystals (T = Fe, Co, Ni). After confirming that these compounds adopt the HoCoGa₅ type, we analyze their electronic structure using density functional theory (DFT) and DFT-calibrated Hückel calculations. The observed valence electron count range of the HoCoGa₅ type is explained in terms of the 18-<i>n</i> rule, with <i>n</i> = 6 for the Ln atoms and <i>n</i> = 2 for the T sites. The role of atomic sizes is investigated with DFT-chemical pressure (DFT-CP) analysis of ScNiGa₅, which reveals negative pressures within the Ga sublattice as it stretches to accommodate the Sc and T atoms. This CP scheme is consistent with HoCoGa₅-type gallides only being observed for relatively small Ln and T atoms. These conclusions account for the relative positions of the HoCoGa₅, BaMg₄Si₃, and Ce₂NiGa₁₀ types in a structure map, demonstrating how combining the 18-<i>n</i> and CP schemes can guide our understanding of Ln–T–E systems

Filling in the Holes: Structural and Magnetic Properties of the Chemical Pressure Stabilized LnMn_<i>x</i>Ga₃ (Ln = Ho–Tm; <i>x</i> < 0.15)

Single crystals of LnMn_<i>x</i>Ga₃ (Ln = Ho–Tm; <i>x</i> < 0.15) were grown from a Ga self-flux. These compounds crystallize in a variant of the AuCu₃ structure type where Mn partially occupies the Ga₆ octahedral holes. Introduction of the Mn guest atoms allows for modulation of the structures and magnetic properties of their hosts: While TmGa₃ orders antiferromagnetically at ∼4.2 K, TmMn_<i>x</i>Ga₃ (<i>x</i> = 0.05, 0.10) remains paramagnetic down to 1.8 K. Ho and Er analogs order antiferromagnetically, with effective moments and Néel temperatures, respectively, decreasing and increasing as a function of Mn concentration. DFT–chemical pressure analysis elucidates the trends in the stability of LnGa₃ AuCu₃-type phases and their stuffed derivatives. Guest atom insertion supports expansion of the filled octahedra, allowing the relief of negative chemical pressures in the surrounding Ga–Ga contacts